ssc-452 aluminum structure design and fabrication guide ship

Aluminum Marine Structure Guide
For stress corrosion cracking, Aluminum Standards and Data-2006 (Aluminum
Association, 2006A) gives an A rating to 5083-H116, 5083-H321, 5086-H111, 5086-H116, and
all tempers of 5454. However, a B rating is given to 5083-H111 and to all tempers of 5456. For
the 6xxx-series, the general corrosion of all tempers of 6063 are given an A rating while all
tempers of 6061, 6082, and 6005A are given a B rating. For stress corrosion cracking resistance,
all tempers of 6063 and the T6 tempers of 6061, 6082, and 6005A are given an A rating. This
information comes from data supplied to the association by its member companies. Much of the
data is proprietary. Note that the H111 tempers refer to extrusions, while the H116 and H321
tempers refer to plate.
This limited data seems to indicate that alloy 6061 is not as suitable for marine use as
some alloys of the 5xxx-series, yet it is ranked similar to 5456, which has seen many years of
satisfactory marine service, although generally used for naval vessels in topside applications
where it is coated. The most successful marine service experience is for the 5086 alloys, which
have been used for the uncoated hulls of workboats and supply boats that have seen more that 30
years of service with little evidence of corrosion.
More data is needed for evaluation of the relative corrosion resistance of aluminum alloys
intended for marine use. Compliance with the new ASTM B928 specification provides
assurance of corrosion resistance for 5xxx-series alloys, but that specification does not apply to
6xxx-series alloys because they have low magnesium content and thus are not prone to the same
sensitization phenomena. Caution is needed in the use of integrally stiffened extrusions that are
being used as deck plating. Extension of that use to areas of the hull that receive continuous sea
water exposure, including wet decks of multi-hulled vessels and side and bottom plating should
not be made until conclusive corrosion testing has been accomplished.
2.2.3 7xxx-Series
The 7xxx-series alloys have zinc as their primary alloying agent, with a small amount of
magnesium added. Some alloys also contain copper or chromium. These alloys are heattreatable
and can acquire very high strengths, with yield strengths as much as 540 MPa (78 ksi)
for alloy 7178-T6. If the alloys do not contain copper, they are weldable, although with a
significantly reduced yield strength, such as alloy 7005-T53, which has a welded yield strength
of 165 MPa (24 ksi). 7xxx-series alloys are used in aerospace applications and for other service
such as automobile bumpers.
One producer developed a variant alloy, designated RA7108.50-T79, which has a welded
yield strength on a 50-mm gage length of 140 MPa (20.3 ksi). Initial testing indicated that the
alloy had good corrosion resistance. (Hval and Sande, 1997). DNV accepted the use of this alloy
in a few high-speed light craft in the period 1996 to 1998, but did not approve its use in larger
vessels before service experience was obtained. The alloy was originally accepted for use in
”dry spaces” internally in ships. In areas where contact with water was expected such as bilge
areas, coating was to be applied. The alloy was not accepted in ballast tanks and other locations
continuously exposed to seawater.
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